The present invention relates to the field of integrated circuit fabrication. More specifically, the present invention relates to the field of transistor fabrication in silicon-on-insulator structures.
Silicon-on-insulator (SOI) technology has become increasingly important in the field of integrated circuits. In SOI fabrication, a layer of semiconductor material overlies an insulating layer; typically, a single crystal layer of silicon overlies a layer of silicon dioxide, which itself overlies a silicon substrate. When sapphire is substituted as the insulating layer, replacing silicon dioxide, a similar technology results, commonly known as silicon-on-sapphire (SOS). Both SOI and SOS devices offer inherent radiation hardness, better high temperature performance, and reduced parasitic capacitance. Consequently, SOI or SOS technology is well suited for high performance and high density integrated circuits.
One problem presented by the use of silicon-on-insulator structures is the effect of the floating body node on the characteristics of field effect transistors. In particular, SOI devices exhibit various floating body effects due to the lack of an effective contact to the body or well region. This floating body, however, provides some advantages to the operation of the device. For example, a floating body region provides higher drive current through the channel region, which provides for faster operation of integrated circuits.
At some point in the operation of the device, it is necessary to contact the floating body of an SOI device. One departure from typical techniques used to contact the floating body was disclosed in U.S. Pat. No. 5,095,348 to Houston. Houston reveals a method and structure which allows for a floating channel region when a voltage is applied which turns the main transistor on and a body-contacting transistor, also referred to as a channel transistor, off. On the other hand, when a voltage to turn off the main transistor is applied, the body-contacting transistor is turned on, thus tying the well to the reference voltage. This tying of the well can only be accomplished, however, if the threshold voltage, V.sub.T, of the body-contacting transistor is carefully set. Unfortunately, carefully setting the appropriate threshold voltage is not easy. Setting this threshold voltage can become an even more complex processing problem if the threshold voltage necessary to tie the well differs from the threshold voltage desired on other channel devices. Furthermore, because the Houston device includes both a main transistor and a body-contacting transistor, it covers a large area.
Bipolar snapback is another problem that reduces the effectiveness of these SOI and SOS transistors. In transistor operation, electrons move across the channel and possess enough energy such that they cause impact ionization of other lattice sites, creating an additional electron-hole pair. This newly created hole attempts to recombine at the source; in the meantime, however, it raises the well potential. This raised potential causes more electrons to be injected. As impact ionization occurs and more electron-hole pairs are created, these holes continue to raise the well potential. Eventually, the transistor enters the snapback state and latches on; in this state, the transistor cannot be shut off unless the power supply is removed.
Accordingly, it is the primary object of this invention to provide contact to a floating body of a silicon-on-insulator transistor which involves less complicated processing and design considerations.
It is another object of this invention to provide a contact to the body which does not require the user to carefully set a threshold voltage.
It is a further object of this invention to provide a transistor that can sufficiently remove the holes so as not to raise the well potential, thus, preventing the device from entering the snapback state.
It is still another object to provide a transistor which covers a smaller area, thus reducing the overall size of the transistor.
It is yet another object to provide a transistor that takes advantage of the fact that when the transistor is fully depleted, this fully-depleted condition can act as a switch for connecting the floating body.
The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.